Page 95 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

MODULE 12 : Induction Motors and Special Motors

LESSON 70-75 : Principle of induction motors

Objectives

At the end of this lesson you shall be able to:
• state the principle of 3 phase induction motor & wound rotar
• explain briefly the method of producing a rotating magnetic field
• explain necessary of starters, types of starters
• explain DOL, manual star Delta, Semi automatic, automatic star Delta, rotor wound & auto transform starters
• explain losses & speed controls of motors.

Working principle of three phase induction motor: An electrical motor is which converts electrical energy into
a mechanical energy. In case of three phase AC operation, most widely used motor is Three phase induction
motor. This type of motor is self-starting induction motor. It works on the same principle of dc motor, that is the
current carrying conductor is placed in magnetic field will tend to create force. The induction motor differs from dc
motor, the rotor of the IM is not electrically connected to the stator, but induces a voltage or current in the rotor by
the transformer action. According to Faraday’s laws an emf induced in rotor circuit due to the rate of change of
magnetic flux linkage As the rotor winding in an induction motor are either closed through an external resistance
or directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar
and due to this emf a current flows through the rotor conductor. Here the relative velocity between the rotating
flux and static rotor conductor is the cause of current generation; hence as per Lenz’s law the rotor will rotate in
the same direction to reduce the cause i.e. the relative velocity.

To reverse the direction of rotation of a rotor: The direction of rotation of the stator magnetic field depends
upon the phase sequence of the supply. To reverse the direction of rotation of the stator as well as the rotor, the
phase sequence of the supply is the changed by changing any two leads connected to the stator.

Production of revolving field

The resultant magnetic field produced by the combined effect of the three currents is shown at increments is
shown at in increments of 60degree for one cycle of the current.
At position 1 the phase currentIr is zero and coil R producing zero flux. the phase current Ib is positive and Iy is
negative. The phase Ib is positive and Iy is negative. Considering the instantaneous current direction of these
three phase windings at position 1 we indicate the current direction. Using Maxwell ‘s cork screw, the resulting
flux by these currents will produce a flux. the arrows show direction of the magnetic field and the magnetic poles
in the stator core. It will continue in position 2,3, etc. It will be seen that for successive increment of 60’0 electrical.

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CITS : Power - Electrician & Wireman - Lesson 70-75